This invention relates to a constant speed driving devices for vehicles, and more particularly to a constant speed driving device for a vehicle with a pressure-charger supplying a pressurized air to the engine during effective operation.
A conventional constant speed driving device of this type has been disclosed, for instance, by Japanese Patent Application (OPI) No. 39311/1983 (the term "OPI" as used herein means an "unexamined published application"). The conventional device is designed as follows: An actual vehicle speed and an aimed vehicle speed are subjected to comparison by using control signals provided by a set switch for specifying a constant driving operation and a resume switch for instructing an increase in vehicle speed, and a signal provided by a vehicle speed sensor. In order to make those vehicle speeds equal to each other, the amount of control of a throttle actuator is calculated by using a control constant. In addition to the accelerator, the throttle actuator to be coupled to the throttle valve is driven, for a constant speed control of the vehicle. The amount of control of the throttle actuator depends on the control constant. Heretofore, the control constant is maintained unchanged irrespective of the operating conditions of a turbo-charger, this is an example of the pressure-charger.
FIG. 8 shows an output characteristic of an ordinary vehicle engine without a turbo-charger, and FIG. 9 shows an output characteristic of a vehicle engine with a turbo-charger. In each of FIGS. 8 and 9, angle parameters indicate the opening degrees of the throttle valve. As is seen from comparison of FIGS. 8 and 9, in the case of the engine with the turbo-charger, its power characteristic (or output torque) is not linear with respect to the opening degree of the throttle valve; that is, the output torque of the engine becomes considerably large as the turbo-charger operates.
Hence, the conventional constant speed driving device provides the following problems: If, in the case of an engine with a turbo-charger, the constant speed driving control is performed by using an ordinary control constant while the turbo-charger is effective, then the output torque of the engine changes greatly. As a result, the vehicle speed is greatly fluctuated from the aimed value, as hunting phenomenon. Accordingly, the vehicle becomes uncomfortable to ride in.
In order to overcome the above-described difficulties, heretofore a method is employed in which the control constant is set to a value with which the amount of control is suitable during the effective operation of the turbo-charger, or it is set to a value between those which are suitable for the effective operation and the non-effective operation of the turbo-charger, respectively. However, the method is still disadvantageous in that, when the turbo-charger is not effective, the amount of control is rather insufficient; that is, the method lowers the control performance.